1. Field of the Invention
The present invention relates to a microprocessor controlled telephone which contains a regulated D.C. voltage supply powered by one or more energy sources, and more particularly to a controller which monitors the power source and produces a digital output to the microprocessor to change the energy source.
2. Background Art
A number of microprocessor controlled telephone instruments have been developed for the consumer and business telephone market. Some units, such as the Answercomm II, manufactured by GTE Communication Systems Corporation, have no test or alarm for low battery voltage. Other units, such as Radio Shack's Duofone 160, require a manual "battery test" button to be pushed to determine battery condition. Other units do not adequately sense or manage certain conditions that could occur, e.g. a lower than normal tip and ring voltage when in the "off hook" state requiring switching to battery back up for operating power and/or memory retention. In the case of 110 volt A.C. line powered units, the phone may not be able to sense the loss of the A.C. power source and switch to a battery back up supply; for example, GTE's Answercomm II. A number may have little or no sensing circuitry such that they may, in fact, draw battery power when not required thereby shortening battery life.
For those circuits which do incorporate threshold sensing, the circuitry may be very complex requiring significant current to operate the sense function. Other types of sensing circuits involve use of operational amplifiers or integrated circuit comparators, which, in turn, require a number of external components resulting in more expense and greater power dissipation than in the present invention. Standard commercially available comparators can be subject to influence from ambient noise or instability under certain conditions. Reliability also may not be as good because of the higher complexity and greater number of parts required as associated with the commercially available comparators. In addition, a certain minimum supply voltage is required for most commercial comparators in integrated circuit form, which introduces a complication if the power supply itself is to be sensed and action is to be taken when a low voltage condition exists.
Most techniques utilized do not appear to adequately address the problem of sensing a low battery condition so that a microprocessor can initiate a corrective action procedure.